The nine-month-old bike-share program in Boston offers 600 bicycles at 61 stations, but the program, called Hubway, lacks one important safety feature: helmets for rent. That could explain why fewer than 1 in 5 bike-sharers are using them, compared with more than half of those who ride their own bikes throughout the city, according to a study published Monday.

Researchers from Beth Israel Deaconess Medical Center observed more than 3,000 bicyclists in Boston and Washington, D.C., including 562 who were riding shared bicycles through city programs. Only 19 percent of those on shared bikes wore helmets, while 51 percent of those riding their own bikes had headgear, the authors reported in the journal Annals of Emergency Medicine.

Pedaling without helmets puts riders at far greater risk of suffering a serious head injury during an accident, the authors said.

“I think it’s a problem of access,’’ said Dr. Christopher Fischer, an emergency room physician at Beth Israel Deaconess and one of the study’s authors. “It’s often more of a spur-of-the-moment decision to use a bike-share, and riders often find themselves without helmets.’’

Overall, a higher percentage of cyclists in Boston rode without helmets than in Washington. During Hubway’s inaugural season, which ran through November, riders took 140,000 trips, and there are 5,600 annual members.

Some Hubway bikers said yesterday that they do not think helmets are necessary to ride a few blocks. Mohammad Marouf, 24, of Boston said he never wears a helmet when he borrows a Hubway bike three to four times a week. “I don’t drive it that far,’’ he said. “I mean, from point A to point B, it’s usually less than five minutes.’’

Carl Robinson, also of Boston, agreed that a helmet is unnecessary for his rides, which he described as “a short jaunt.’’

About 51,000 bike-related head injuries and 630 deaths occurred in the United States in 2009, the most recent year for which national data were available. Research suggests that wearing helmets could decrease the risk of head injury and brain injury by 65 to 88 percent.

“There’s a wide spectrum of injuries that can occur from falling off your bike and hitting your head, from a minor bump to a severe, traumatic devastating injury’’ that causes permanent paralysis, cognitive defects, or, in rare cases, death, Fischer said. Although helmets cannot protect against every kind of accident, like “getting creamed by a bus,’’ he added, “they provide a degree of protection, and anything is better than nothing.’’

Massachusetts law stipulates that bike riders 16 years of age or younger must wear a helmet, but, like other states, it has no law requiring teenagers and adults to wear helmets.

One solution could be helmet-rental kiosks set up at the rental bike stations. Last year, undergraduates from MIT developed a prototype apparatus they dubbed HelmetHub that would dispense bike helmets, adjustable to fit most head sizes, through a touch-screen vending machine and would sanitize returned helmets.

The city of Boston plans to test the machine this summer at one of the busier stations, such as South Station or North Station. “We’re still working out the specifics,’’ said Kristopher Carter, interim director of the city’s Boston Bikes program. Riders might pay $5 to borrow a helmet and get a refund upon its return.

Whether that would work to improve helmet usage remains to be seen. Helmet-less Hubway rider Yen Lin, 40, of Boston said he probably would not don the headgear, even if it was offered next to a Hubway station. “The helmet would destroy my hair’’ during the daily work commute, he said.

But Ted Wu, a 34-year-old tourist from Los Angeles, said he would have welcomed a helmet rental machine when he borrowed a Hubway bike Monday, adding that he decided to ride without a helmet after succumbing to pressure from friends.

For now, tourists and others can download an app from www.thehubway.com, the Boston Bikes’ website, which provides riders with locations of dozens of convenience stores that have joined with the city to sell bike helmets for $8.

To increase helmet use, the Boston Public Health Commission plans to kick off a public awareness campaign this summer that could take the form of print ads posted around the city and helmet giveaways.

“We’re trying to determine a target audience for this campaign,’’ said spokesman Nick Martin. “Perhaps young men, since they’re the most frequent riders and experience the most frequent injuries.’’

In the new study, men were 60 percent more likely than women to ride without helmets.

In 2011, the commission distributed 2,260 helmets for free or for a $5 fee.

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